Judge Robert Bork is down - but by no means out. President Reagan's highly controversial nominee to the United States Supreme Court came down from the witness stand of the Senate Judiciary Committee Saturday after five days of sometimes grueling interrogation. Many long-time observers say the questioning has been more probing than for almost any previous aspirant to the high court. Some of Judge Bork's supporters - including Republican Senators Orrin Hatch of Utah and Alan Simpson of Wyoming - suggested that some of their colleagues' remarks turned the hearings into more of an inquisition than a fact-finding mission.

Senator Simpson insisted that Bork was the target of liberals and other opponents of the White House's social and economic philosophy even as early as last fall, after the confirmations of William Rehnquist as chief justice and Antonin Scalia as an associate justice.

``When Powell [Associate Justice Lewis Powell, whom Bork would replace if confirmed] quit ... they got ready for the struggle. It was a warning to the President. And when Bork was nominated, the package was detonated,'' Simpson said. (See story on Page 4.)

The nominee's critics - notable among them Democratic Senators Edward Kennedy of Massachusetts and Howard Metzenbaum of Ohio - saw it very differently. From the start, Senator Kennedy opposed the appointment, charging that Bork would set civil rights in the courts back a half-century and that this nomination is a way for the White House to achieve its long-fought-for reactionary social agenda.

Amid this partisanship, a clearer profile of Robert Bork emerged from the hearings. Not one but two Borks seemed to surface.

The first: an outspoken, if not rebellious, legal scholar and intellectual who has been anxious to challenge generally-accepted judicial doctrine, even precedential rulings of the Supreme Court.

The second: a right-of-center - but not outside an acceptable ideological spectrum - jurist and staunch defender of the Constitution who would follow court precedent when it challenges his own beliefs.

The first Robert Bork, opponents fear, might apply his philosophy as a member of the Supreme Court to try to reverse long-standing decisions on antitrust, abortion, affirmative action, and rights of the accused.

The second, on the other hand, might vote to curtail what he considers sweeping judicial mandates in these areas - but he would heed precedent and not upset ``settled law.''

At this point, the latter Bork would seem to stand a much better chance of Senate confirmation than the former.

The portly, graying, bearded high-court nominee tried hard to convey the second image before the 14-member Judiciary Committee. He stressed that he would support individual and civil rights and follow precedent in his decisions. Judge Bork said he would be ``disgraced in history'' if he did something as a justice other than what he said he would do during the hearings. The witness sometimes reacted vehemently to what he considered a hostile question - particularly by Senator Kennedy, his most vocal opponent. But he never lost his temper and seldom his composure.

Now Bork defenders and detractors will appear before the committee for the next several days.

The committee's recommendations to the full Senate will not likely come before the Supreme Court starts its fall term Oct. 5. Meanwhile, the Senate panel seems as divided as it was before last week's testimony - with five Republicans for, five Democrats against, and four senators (three Democrats and one Republican) undecided.

Key questions still linger. Among them: How far will the nominee go in embracing the judicial concept of stare decisis, deciding cases in accordance with previous decisions and constitutional interpretations? Will his devotion to the theory of ``original intent'' allow him to take into account emerging ideas of social justice not specifically spelled out by the Constitution's founders? Would his commitment to judicial restraint, and deference to the president and Congress, be tempered when other branches of government clearly violate individual rights?

Judge Bork has repeatedly insisted that he has ``no ideological agenda'' and his first priority is ``faithfulness to the Constitution.'' President Reagan has a lot riding on the Bork nomination. But so do Democrats who are anxious not only to embarrass the President politically but also to nip in the bud the possibility that he might accomplish with this appointment a social agenda rejected by Congress and so far rebuffed by the courts.